A gain tilt amplifier is useful in microwave and other communication systems for providing a gain response with a tilt that can be varied across a frequency band. This is particularly valuable if the amplifier can be realized in monolithic or MIC form. This allows an overall complex circuit to be electronically tuned after construction to meet tight gain flatness specifications. It is particularly desirable to have such an amplifier that has minimal effect on circuit noise and power output.
These features are provided in the present invention by providing a variable feedback impedance for a base amplifier. In particular, such a gain tilt amplifier circuit includes a base amplifier connected between an input terminal, for receiving the input signal, and an output terminal for transmitting the amplified input signal.
A feedback circuit couples the base amplifier output to the amplifier input for feeding back a portion of the amplified input signal to the input. The feedback circuit includes an inductor coupled in series with a variable resistance. The variable resistance is responsive to the control signal for producing a resistance determined by the control signal. The gain of the base amplifier is determined over the predetermined frequency range by the impedances of the inductor and resistance.
In the preferred embodiment of the invention, the base amplifier is a base field-effect transistor (FET). The feedback inductance is taken from a portion of a coil coupling the drain of the base FET with the output terminal. The variable feedback resistance is provided by a feedback FET having a gate for receiving a control signal for varying the resistance between the source and drain of the feedback FET. To increase linearity, a resistor is connected between the drain and source of the feedback FET.
Thus, the feedback impedance, and therefore amplifier gain, is variable. Additionally, the feedback inductance varies the overall feedback impedance with frequency, producing higher impedance at higher frequencies. The relationship between the inductance and resistance in the feedback circuit determines the tilt of the overall gain response.
These and other features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiment and associated drawings.